Recent years have seen technical advances in portable electronic devices, hybrid vehicles, etc., and there has been a growing demand for batteries (in particular, secondary batteries, such as lithium-ion secondary batteries) with a higher capacity for use in those devices and vehicles. However, the development of high-capacity cathodes for lithium-ion secondary batteries currently lags behind that of high-capacity anodes. Even actively researched and developed high-capacity Li (Ni,Mn,Co)O2-based materials only have a capacity of about 250 to 300 mAh/g.
Sulfur, which has a theoretical capacity of as high as about 1670 mAh/g and has abundant resources, and which is inexpensive, is one of the promising high-capacity electrode materials. However, elemental sulfur has low conductivity. Further, in battery systems using an organic electrolyte (e.g., lithium-ion secondary batteries), lithium polysulfide generated during the charge and discharge process is dissolved into the electrolyte solution and precipitated on the anode etc., causing the problem of capacity reduction.
To solve this problem, a variety of attempts have been made by forming a composite of elemental sulfur with various organic materials, such as resins and pitch, imparting conductivity to the composite, and inhibiting dissolution and diffusion of lithium polysulfide into the electrolyte solution (e.g., Patent Literature (PTL) 1, PTL 2, and PTL 3, Non-Patent Literature (NPL) 1, NPL 2, and NPL 3). These documents report that the above sulfur-carbon composites exhibit a relatively high capacity and relatively excellent cycle characteristics. Until now, these sulfur-carbon composites have been produced by using, as a starting material of the carbon source, carbon materials, such as porous carbon; polyacrylonitrile (PAN); pitch; or other solid organic substances, and heating the starting material with elemental sulfur or with a sulfur-containing starting material. In particular, an organic sulfur material produced by using PAN as a starting material is considered to be a promising candidate as an electrode material that undergoes less cycle deterioration.